Satellite infrared window channel measurements around the 11 to
12 micron band have long been used to estimate precipitation over
the tropics. The availability of IR window channels on board
geostationary satellites, which see through to the surface except
when clouds are present, provides high temporal sampling (3-hourly
or better) from space. Since clouds are opaque at IR frequencies,
the resulting brightness temperatures over cloudy regions
corresponds to the temperature of the cloud top, providing a
measure of the cloud-top height. In the tropics where deep
convection is prevalent, high-level cloudiness is reasonably well
correlated with precipitation, at least when averaged over large
space and time scales. Unfortunately the relationship between
cloud-top height and precipitation changes with latitude and over
land as well as over regions with significant amounts of
non-precipitating cirrus or other clouds. As a result, IR only
techniques suffer from significant regional and time dependent
biases, which is why the merged techniques use passive microwave
rain estimates to "adjust" the IR estimates.

The Outgoing Longwave Radiation (OLR) Precipitation Index (OPI)
is computed from NOAA polar-orbiting IR window channel data using
the technique developed by Xie and Arkin [1998]. The OPI is
produced by calibrating OLR values using rain estimates from
the CPC Merged Analysis of Precipitation (CMAP) data [Xie and
Arkin [1996]. The OPI dataset is one of the component rainfall
estimates used in the merged CAMS-OPI dataset [Janowiak and
Xie, 1999].